1. Field of the Invention
The present invention generally relates to a method and an apparatus for increasing a data transfer rate of a mobile telecommunication system, and in particular, to a method and an apparatus for increasing a data transfer rate using an Adaptive Modulation and Coding (AMC) scheme together with a control of a transmitting power in a multi-channel system using a plurality of antennas.
2. Description of the Related Art
The quantity of data provided from mobile telecommunication systems has been rapidly increased with the development of mobile telecommunication system technology. Mobile telecommunication business companies have developed technology which enables high-speed data transmission in order to provide high-quality services to users.
As an example of a general method of transmitting and receiving data, a fixed modulation scheme is used to transmit data through an antenna, as shown in FIG. 1A, where data is transmitted using a modulator and is received using a demodulator.
If the data is transmitted using the fixed modulation scheme as illustrated in FIG. 1A, a transfer rate of the data is constant even in a good state of a channel. For example, if a 4 Quadrature Amplitude Modulation (4QAM) scheme is used, a number of transmittable bits per a symbol is constant, i.e., 2 bits, with an increase of a Received Signal Strength (RSS) as shown in FIG. 1B.
In order to solve the above-described problem, an AMC scheme is used to transmit data. The AMC scheme changes a modulation scheme and a coding rate of a channel coder according to changes of channel environments of a downlink. As shown in FIG. 1C, a modulator and a demodulator show how a portable terminal measures a Signal-to-Noise Ratio (SNR) of a downlink and transmits information as to the SNR to a Base Station (BS) through an uplink. The BS estimates environments of a downlink channel based on the information and designates an appropriate modulation scheme and a coding rate of a channel coder based on the estimated value.
FIG. 1D is a graph illustrating a data transfer rate measured if the AMC scheme is used. If a maximum 512QAM modulation scheme is used, data of 9 bits may be transmitted per symbol. Thus, the data transfer rate is higher than when a fixed modulation scheme is used.
Also, there is a method of improving multi-path fading by which a transmitter can transmit data through a plurality of antennas to a plurality of paths, and a receiving portable terminal can reduce interference among the data received through the plurality of paths. An apparatus using the method is illustrated in FIG. 1E, where the transmitter uses two modulators and the receiver uses one demodulator.
The method of FIG. 1E is not to increase a data transfer rate using “Alamouti coding” but to increase an SNR in order to increase a gain. The method is well known and a description will be omitted.
If an AMC scheme is used among the above-described data transmission methods, a large number of bits may be included in a symbol and then modulated to increase a data transfer rate. However, an SNR must be increased to maintain a constant Bit Error Rate (BER).
Also, if a RSS of a portable terminal is smaller than a minimum RSS at which data of one bit is communicated per one symbol using a Binary Phase-Shift Keying (BPSK) method, the data cannot be transmitted through a plurality of antennas.
An AMC scheme and a method of transmitting and receiving data using a multi-path are independent techniques for increasing a data transfer rate of a transmitter. However, if the two methods are combined and used, a performance of the transmitter can be greatly improved.